Numerical combination locks are known, for example from DE 28 16 969 A1 or from DE 30 29 735 C2, in which the opening combination can be set mechanically by means of several tumbler plates, whereby by turning of the turning knob, all of the tumbler plates eventually become arranged in such a way that a finger located on the drop lever finally matches recesses in the tumbler plates in such a way that the drop lever is no longer held up, but instead, when the finger enters these recesses, the drop lever can also enter the drop opening in the cam plate. In this case, there exists the danger that, due to user error, the user will have to select the opening combination several times in order finally to be able to open the lock.
For that reason, there is known from U.S. Pat. No. 4,745,784 a tumbler lock in which the opening combination is contained in an electronic memory, and the turning knob is used to operate the actual values of the opening combination in the correct sequence by means of individual switches so that the comparison can take place between the setting selected by the turning knob and the stored value. If the values agree, the drop lever is brought into the coupling position of the cam plate by means of electrical operation, whereby, in the case of this previously known solution, the drop lever is held against a lateral finger with the aid of an additional plate, which, after the correct opening combination is selected, is turned in such a way that the drop lever can drop into the drop opening of the cam plate.
In addition, it is known from WO 92/11430 that a sliding bar can be provided to force the drop lever to swivel into the drop opening of the cam plate. This sliding bar is displaced by a turning of the turning knob and at the same time acts upon a lever arm, which, for its part, is joined with the drop lever and then swivels this drop lever. In conjunction with this, this sliding bar is to be pushed against a resetting spring, which pushes the sliding bar back into its original position after the operation of the drop lever, so that the sliding bar is again available for operating the drop lever the next time the opening procedure is carried out.
If, however, the resetting spring of the sliding bar malfunctions or even breaks, or if, for example after a certain amount of wear, the sliding bar becomes jammed in the position it is in at the instant of the coupling of the drop lever, it will not be returned to its original position.
In spite of that, however, the user can still close the lock again by means of a reverse direction turning movement of the turning knob, whereby this locking takes place independently of the sliding bar that was mentioned. If the lock is now to be opened once again, it is no longer possible because the sliding bar is not in its original position, from which it is able to move the drop lever into the drop opening.